The present invention is directed to control systems for electric motor driven traction vehicles and, more particularly, to a control system which provides a dynamic drag effect when a vehicle speed control pedal is released.
Electric motor driven traction vehicles such as large earthmoving machines typically comprise a prime mover driving a generator or alternator which provides electrical energy to high horsepower electric motors connected in driving relationship to wheels of the vehicles. The prime mover is commonly a diesel engine and the electric motors are generally variable speed reversible direct current motors. A vehicle operator controls the vehicle speed and direction of travel, i.e., forward or reverse, by manipulation of a speed control pedal and a forward-reverse selector lever. The speed control pedal is adapted to control the diesel engine speed (RPM) which controls the power output of the generator or alternator thus varying the energy supplied to the electric motors. The forward-reverse selector lever is usually adapted to operate switches to control the direction in which current passes through the motor field windings.
In prior art electric propulsion systems, a desired reduction in speed of the vehicle is accomplished by releasing the speed control pedal and either allowing the vehicle to coast or activating the mechanical or the electrical braking system of the vehicle. Releasing the speed control pedal allows the prime mover to slow thus reducing the generator terminal voltage. When the generator terminal voltage becomes less than the CEMF of the traction motor armature, current reverses and flows from the motor armature to the generator. The motor field excitation circuit responds to the reduction in current flow from generator to motor and correspondingly reduces field excitation, achieving zero field excitation at substantially the same time as armature current reverses. With field excitation removed the motors free-wheel, i.e., removing field excitation is equivalent to disengaging the clutch in an internal combustion engine driven automobile with a standard transmission. If the vehicle is proceeding down a slight incline, removing excitation to the motors may not reduce the forward speed since the motors free-wheel. In addition, the lack of any traction motor resistance to rotation allows the vehicle forward momentum to remain substantially constant and results in an undesirable "feel" to the operator as compared to a non-electric vehicle which presents a drag effect when the speed control pedal is released due to the wheels causing the engine and transmission parts to continue to rotate.